Fossil fuel power plants can be fitted with carbon dioxide (CO2) capture equipment either when first built or as a retrofit. One technology that shows near-term promise for large-scale practical application comprises processes in which ammonia-rich solutions and slurries are used to absorb CO2 from power station flue gases.
For example, published International patent application no. WO 2006/022885 A1 (E. Gal) describes a process of the chilled ammonia type, in which downstream of conventional pollution control equipment, such as electrostatic precipitators and flue gas desulphurisers, the flue gases are cooled to well below ambient saturation temperature and subject to wet scrubbing with cold water to remove residual contaminants not removed by the conventional equipment. This is followed by passage of the cleaned flue gases through one or more CO2 absorbers operating at about atmospheric pressure and at a low temperature in the range 0° C. to 20° C., where the gaseous CO2 in the flue gases is reacted with an ammoniated solution. Cooling of the flue gas before passage through the CO2 absorber reduces its moisture content and its volume, thereby increasing the CO2 concentration and making its capture more efficient. Moreover, the relatively low scrubbing temperature in the absorber enhances mass transfer of CO2 to the ammonia solution while reducing the ammonia's vapour pressure, so tending to prevent its evaporation into the flue gas stream. The CO2-rich ammonia solution from the absorber vessel is then pressurised and heated to release the absorbed CO2 as a relatively clean pressurised CO2 gas stream containing low concentrations of ammonia, the latter being recovered by a further cold scrubbing process. The ammonia solution is now CO2-lean, and is re-circulated for further CO2 absorption duty. Meanwhile, the CO2 gas stream can be cooled and further pressurised for storage and sequestration. The reader is referred to the above-mentioned patent publication for a more complete description of the process.
Carbon capture processes are of course intended to slow down the onset of climate change caused by increased concentrations of greenhouse gases in the atmosphere. However, most climate specialists agree that some global warming is already occurring and will continue for several decades, at least. The results of global warming-driven climate change will include increased aridity in some regions of the world, with concomitant water shortages. Additionally, there will be an increased need for fertilisers to boost crop yields.